1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of the Related Art
Hitherto, a sheet conveying apparatus for conveying the sheet is provided in each of an offset printing apparatus, a printer using an electrophotographic system or an ink jet system, a copying apparatus, and an image forming apparatus for forming an image at a predetermined position on a facsimile (FAX) sheet.
In such a sheet conveying apparatus, there is a case where high precision is required in a sheet conveying position and a sheet conveying speed. In such a case, various kinds of control such as high precision sheet conveying speed control and feed amount control are necessary. In the sheet conveying apparatus in the related art, control using a plurality of sheet detecting units (sensors) is generally made in many cases. The sheet conveying apparatus which needs such high precision sheet conveying speed control and feed amount control is used in, for example, an inspection system for detecting a specific mark on the sheet (for example, a hologram or the like on a bill).
As an image forming apparatus having such a sheet conveying apparatus which needs the high precision sheet conveying speed control and feed amount control, for example, there is an apparatus having a function for printing images onto both sides of the sheet. According to such an image forming apparatus, in the case of forming the images onto the both sides of the sheet, the recto and verse sides of the sheet in which the image has been formed on the first surface are reversed and the sheet is fed to an image forming unit (image transfer unit).
As a system for reversing the recto and verse sides of the sheet, there is a system using what is called a switch-back system. In the switch-back system, after the sheet passed through a fixing apparatus for fixing the image onto the sheet, the sheet is temporarily pulled in a reverse conveying apparatus and guided to a duplex conveying apparatus. Since the switch-back system has a simple construction as a system for reversing the sheet and is advantageous in terms of a space, it is used as a general system in many cases.
In the switch-back system, a reference of a sheet conveying direction, that is, a leading edge and a trailing edge are exchanged. Therefore, even in the case of an apparatus having a construction in which a skew feed correcting ability of a skew feed roller system is excellent, a positional deviation of the recto and verso images in the sheet conveying direction occurs.
There is a variation of sheet dimensions due to a cutting variation or a fixing heat contraction variation which depends on a fibrous texture. If timing for a toner image and timing for a leading edge of the sheet are merely uniformly matched by using the sheet leading edge as a reference, the position of the recto image and the position of the verso image are deviated. In the case where the positions of the recto and verso images are deviated in this manner, after the images were formed on both sides of the sheet, the images are partially dropped out during a processing step of trimming or folding or, contrarily, a blank is inserted to a next page. Thus, quality of a printed material deteriorates.
To solve such a problem, detecting units (measuring positions) are provided at two positions on a duplex conveying path and a sheet conveying speed and a sheet length are obtained from pass signals of the sheet detecting units (sensors) (refer to Japanese Patent Application Laid-Open No. 2007-004137).
By detecting the length of sheet S which passes, when the image is formed onto the second surface (verse side), even if the leading edge and trailing edge of the sheet are exchanged, a reference edge at the time of forming the image onto the first surface (recto side) can be recognized. Thus, the position of the image formed on the first surface (recto side) can be also recognized. Therefore, by forming the image of the second surface (verse side) so as to be matched with the image position on the first surface, the occurrence of the positional deviation between the recto and verse images can be reduced.
FIG. 15 is a plan view of two detecting units (measuring positions) S1A and S2A provided for the sheet conveying apparatus in the related art as mentioned above. Two sheet detecting sensors SN1 and SN2 are provided for the first detecting unit S1A on the upstream side in the sheet conveying direction so as to be symmetrical around a center C in the lateral direction which perpendicularly crosses the sheet conveying direction on a sheet conveying path (hereinbelow, such a center is referred to as a conveyance center). Likewise, two sheet detecting sensors SN3 and SN4 are provided for the second detecting unit S2A on the downstream side in the sheet conveying direction so as to be symmetrical around the conveyance center C.
When the sheet S which is conveyed in the direction shown by an arrow in the diagram passes through the first and second detecting units S1A and S2A, detection signals illustrated in FIG. 16 are derived from the sheet detecting sensors SN1 to SN4, respectively. In FIG. 16, T1 and T2 denote sheet leading edge detecting times of the sheet detecting sensors SN1 and SN2 of the first detecting unit S1A; T3 and T4 denote sheet leading edge detecting times of the sheet detecting sensors SN3 and SN4 of the second detecting unit S2A; T1′ and T2′ denote sheet trailing edge detecting times of the sheet detecting sensors SN1 and SN2; and T3′ and T4′ denote sheet trailing edge detecting times of the sheet detecting sensors SN3 and SN4.
When a time during which the leading edge of the sheet S passes between the sheet detecting sensors SN1 and SN3 is assumed to be f, the time f is calculated as f=T3−T1. When a time during which the leading edge of the sheet S passes between the sheet detecting sensors SN2 and SN4 is assumed to be e, the time e is calculated as e=T4−T2.
When a time during which the trailing edge of the sheet S passes between the sheet detecting sensors SN1 and SN3 is assumed to be h, the time h is calculated as h=T3′−T1′. When a time during which the trailing edge of the sheet S passes between the sheet detecting sensors SN2 and SN4 is assumed to be g, the time g is calculated as g=T4′−T2′.
The sheet conveying speed can be obtained by the passing times obtained as mentioned above and a distance D between the first and second detecting units S1A and S2A. An influence of a conveying roller 5 provided on the upstream side of the first detecting unit S1A and an influence of a conveying roller 6 provided on the downstream side of the second detecting unit S2A are averaged. Therefore, a sheet conveying speed V is calculated by the following equation (1) by using an average value Avg(e, f, g, h) of the above times e to h as a passing time of the distance D.V=D/Avg(e,f,g,h)  (1)
Further, when a time during which the whole sheet passes through the sheet detecting sensor SN1 is assumed to be a, the time a is calculated as a=T1′−T1. When a time during which the whole sheet passes through the sheet detecting sensor SN2 is assumed to be b, the time b is calculated as b=T2′−T2. When a time during which the whole sheet passes through the sheet detecting sensor SN3 is assumed to be c, the time c is calculated as c=T3′−T3. When a time during which the whole sheet passes through the sheet detecting sensor SN4 is assumed to be d, the time d is calculated as d=T4′−T4.
Similarly, in order to average those errors, a length L of sheet S is calculated by the following equation (2) by using an average value Avg(a, b, c, d) of the above times a to d as a passing time of the whole sheet.L=V×Avg(a,b,c,d)  (2)
By arranging the two sheet detecting sensors SN1 and SN2 for the first detecting unit S1A and arranging the two sheet detecting sensors SN3 and SN4 for the second detecting unit S2A, that is, by arranging the four sheet detecting sensors in total for them, the length of sheet conveyed at a certain skew feed angle θ can be also measured without providing any special skew feed correcting apparatus.